Device and method for the production of monocrystalline or multicrystalline materials, in particular multicrystalline silicon

ABSTRACT

The invention relates to a device and a method for the production of monocrystalline or multicrystalline materials using the vertical-gradient-freeze method, in particular silicon for applications in photovoltaics. According to the invention a low amount of wastage is achieved in that the cross section of the crucible is polygonal, in particular rectangular or square-shaped. Disposed around the circumference of the crucible there is a flat or planar heating element, in particular a jacket heater, which generates an inhomogeneous temperature profile. This corresponds to the temperature gradient formed in the centre of the crucible. The heat output of the flat heating element decreases going from the top end to the bottom end of the crucible. The flat heating element comprises a plurality of parallel heating webs, extending in a vertical or horizontal meandering course. The heat output from the webs is set by varying the conductor cross section. To avoid local overheating in corner areas of the crucible, constrictions of the cross section are provided at inversion zones of the meandering courses of the webs. The flat heating element can be formed from a plurality of interconnected individual segments.

The present application claims the priority of German Patent ApplicationNo 10 2006 017 621.9-43, filed on 12 Apr. 2006, with the title “Methodfor the production of monocrystalline or multicrystalline materials, inparticular multicrystalline silicon”, the whole content of which isincluded herein by way of reference for purposes of disclosure.

FIELD OF THE INVENTION

The present invention relates generally to the production ofcomparatively large monocrystalline or multicrystalline material blanksusing the vertical-gradient-freeze method (hereinafter also called theVGF method), in particular multicrystalline silicon for applications inphotovoltaics, monocrystalline fluoride crystals and monocrystallinegermanium crystals.

BACKGROUND OF THE INVENTION

Solar cells should have the highest possible degree of efficiency forthe conversion of solar radiation power into electrical current. This isdetermined by a plurality of factors, such as, inter alia, the purity ofthe starting material, the penetration of impurities duringcrystallisation from the contact surfaces of the crystals with thecrucible into the crystal interior, the penetration of oxygen and carbonfrom the surrounding atmosphere into the crystal interior and also bythe growth direction of the individual crystal grains.

A common feature of all known production methods in which a largequantity of molten silicon is solidified to form an ingot is the factthat heat is withdrawn from the crystal melt from its base and hence acrystal grows from the bottom upward. Due to the typically high rate ofsolidification and the absence of a seed crystal, the crystal does notgrow as a monocrystal but is multicrystalline. A block is formedcomprising a plurality of crystal grains of which each grain grows inthe direction of the locally prevailing temperature gradients.

Now, if the isotherms of the temperature field in the silicon melt arenot planar and do not run parallel to the base of the crucible, i.e.horizontally, no planar phase interface forms and the individual grainsdo not grow parallel to each other and vertically from the bottomupward.

This is accompanied by the formation of linear crystal imperfectionseven within the monocrystalline regions. These undesired crystalimperfections can be made visible as etched pits by etching polishedsurfaces (e.g. on silicon wafers). A high number of linear crystalimperfections as described above therefore results in a higher etchdensity.

The minimisation of the density of etch pits, which may be influenced bya plurality of factors, inter alia the establishment of a planar phaseinterface has been a well-known requirement for a long time. The densityof etch pits is therefore a measure of the success in the achievement ofa pillar-type growth of the Si grains by means of a planar phaseinterface. Since the establishment of the HEM method (heat exchangemethod) as the first method suitable for mass production, attempts havebeen made to avoid the drawback of an almost punctiform heat sink on thebase of the crucible (as is known, for example, from U.S. Pat. No.4,256,530) and to achieve a vertical heat flow from the top downward inthe molten silicon.

There are therefore a variety of solutions, which aim, as a first step,to create a heat sink that extends over the entire surface of thecrucible base (see, for example, EP 0 631 832, EP 0 996 516, DE 198 55061). The present invention is based on the assumption that a planarheat sink of this kind is provided.

To produce solar cells as inexpensively as possible, there is a furtherrequirement for the entire silicon ingot to be available for furtherprocessing if at all possible. However, the production process issubject to restrictions. This is due on the one hand to the inwarddiffusion of impurities from the crucible wall into the silicon meltwhile on the other hand the segregation results in an accumulation ofimpurities on the upper side of the silicon ingot so that it isregularly necessary to remove edges of the silicon ingot. A furtherrestriction is represented by the generally rectangular basic shape ofsolar cells. This makes it necessary to cut the silicon ingot to thedesired cross section. In this regard, it is desirable for the amount ofwastage to be kept as low as possible.

The production of multicrystalline silicon from a melt consumes muchenergy. Therefore, there is a further requirement for the capacity ofthe smelting furnace to be used to the optimum degree and to haveeffective thermal insulation. For reasons of space, the base area of themelting crucible should occupy as much as possible of the base area ofthe smelting furnace.

Due to the high economic importance of the production of silicon as astarting material for the production of semi-conductors andsemi-conductor components, a plurality of different approaches forgrowing silicon monocrystals or multicrystalline silicon are known fromthe prior art. For example, U.S. Pat. No. 4,256,530 discloses a methodfor growing a silicon monocrystal using a melting crucible withtwo-layer walls so that the silicon melt does not come into directcontact with graphite or elemental carbon that would otherwise diffuserapidly into the silicon melt.

To obtain the lowest possible dislocation density in the crystal, duringthe crystal growth, care should be taken to ensure that the phaseinterface between solid and liquid is as planar as possible and runstransverse to the direction of crystallisation. This objective requiresthe radial heat radiation to be kept as low as possible. According to WO01/64975 A2, to form a planar phase interface between the base of amelting vessel and its upper opening, a vertically extending axialtemperature gradient is applied and measures are taken to avoid heatdissipation through the side walls of the melting vessel. To this end,all heating elements are enclosed in a jacket of insulating materialsurrounding the melting vessel as a way of preventing an undesirable anduncontrolled heat flow. To this end, a jacket of insulating material isdisposed between the jacket heater and the crucible as an additional wayof preventing a radial heat flow. This achieves a dominance of the axialtemperature profile created by the upper heater and bottom heater.

EP 1 147 248 B1 discloses a device for producing a monocrystal bygrowing the monocrystal from a melt, wherein the furnace has arotationally symmetrical design and wherein a wedge-shaped thermalinsulation is provided around the melting vessel, viewed in thelongitudinal direction of the vessel, with an insulating effectdecreasing going from the upper heater to the bottom heater. As aresult, heat losses close to the bottom heater are greater than thoseclose to the cover heater. This supports a temperature gradient in thelongitudinal direction of the melting vessel that is determined bydifferent temperatures of the upper heater and bottom heater. Thethermal insulation also significantly restricts the heat flow in theradial direction of the melting vessel resulting in the formation ofplanar phase interfaces.

DE 102 39 104 A1, corresponding to US 2004/0079276 A1, discloses acrystal growing furnace for a VGF method or vertical Bridgman method.Two jacket heaters or flat, planar heating devices are disposed aroundthe melting vessel coaxially and vertically one above the other. Inaddition, measuring devices are provided to determine radial temperaturedifferences in the space between the jacket heaters and the meltingvessel. A regulator sets the heat output of the jacket heaters in such away that the temperature difference measured in the radial directionbecomes zero. In this way, planar phase interfaces are establishedresulting in the production of high-quality, low-dislocation siliconmonocrystals.

According to the prior art, the heaters and the external contour of acrystallisation system for multicrystalline silicon are usuallyrotationally symmetrical, i.e. they have a circular profile. Since theusual square-shaped crucible is surrounded by this circular heater, theproblem of corner overheating occurs. This results in thermal stressescausing flaking in the corners and consequently a comparatively largeamount of wastage, which it is desirable to avoid. Typically, with theproduction of large fluoride monocrystals and of germanium crystals,round crystals are produced in round crucibles. The crucibles aresurrounded by round heaters that display no differences between theupper and lower regions as far as thermal radiation is concerned.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a deviceand a method for the inexpensive production of high-quality,low-dislocation monocrystalline or multicrystalline material blanksusing the VGF method, in particular multicrystalline silicon, largefluoride monocrystals or germanium monocrystals.

This object and other objects are achieved according to the presentinvention by a device with the features according to claim 1 and by amethod with the features according to claim 16. Other advantageousembodiments are the subject matter of the related dependent claims.

Accordingly, the present invention relates to a device with a fixedcrucible and a heating device for melting the silicon in the crucible.The heating device and/or any thermal insulation for the device arehereby so designed to create a temperature gradient in the longitudinaldirection in the crucible. This is normally achieved by keeping the baseor bottom of the crucible at a lower temperature than its upper end. Inaddition, with a device of this kind, the heating device is a flat orplanar heating element (hereinafter referred to as a “jacket heater”) tosuppress a heat flow perpendicular to the longitudinal direction, i.e.directed horizontally outward.

According to the invention, the jacket heater or flat, planar heater isa single-zone heater, which is configured such that its heat outputdecreases in the longitudinal direction going from the top end to thebottom end in order at least to contribute to the maintenance of thetemperature gradient formed in the crucible. In other words, by varyingthe heat output of the jacket heater in the longitudinal direction ofthe crucible in a continuous or discrete way, the formation of apredetermined temperature gradient in the crucible is at least assisted.This temperature gradient is established in the melting crucible bydifferent temperatures of an upper heater and a bottom heater in a waythat is known per se. In this regard, the temperature of the bottomheater on the base or bottom of the melting crucible is lower, inparticular below the melting temperature of the silicon to be processed.Hereby, expediently, the bottom heater does not necessarily extend overthe entire base area of the crucible. Although the formation of a planarphase interface in the material to be crystallised, for example silicon,can be realised most precisely with a bottom heater extending over thebase area of the crucible, a planar phase interface sufficient forpractical applications can also be achieved by disposing a cruciblemounting plate between the heater and the crucible permitting thedisposal of a smaller-area bottom heater combined with a cooling device.According to the invention, the temperature gradient between the top andbottom is now reproduced by the heat output of the jacket heater whichvaries in the longitudinal direction of the melting crucible so thatover the entire cross section of the crucible, in particular also in theregion of the corners of the polygonal crucible, a planar phaseinterface forms between the already crystallised silicon and the stillmolten silicon, that is, a horizontally extending phase interface.Therefore, according to the invention, no expenditure is required forthermal insulation between the crucible and the jacket heater becausethe graphite crucible surrounding the quartz crucible is sufficient tohomogenise the temperature profile generated by the jacket heater. Inthis regard, sufficient homogenisation means in particular that, as aresult of the high thermal conductivity of the external cruciblematerial, graphite, local differences relating to the heat radiated bythe jacket heater are equalised. The vertical temperature profile formedin this way in the graphite crucible wall is transferred by the cruciblewall of the poorly heat-transferring quartz crucible through to theinternal wall of the quartz crucible virtually unchanged. At the contactsurface between the molten silicon and the quartz crucible, thetemperature decreases monotonously and approximately linearly from thetop downward. As a result, despite the omission of the layer of thermalinsulation material, it is possible to ensure a planar, horizontal phaseinterface between the crystallised silicon and the still molten silicon.With the same external dimensions of the crystallisation system, thisfacilitates an overall larger crucible cross section and hence,according to the invention, the provision of larger silicon ingotsresulting in significant cost advantages. The single-zone jacket heateraccording to the invention with a defined adjustable temperature profileover the jacket height is particularly advantageous with the productionof multicrystalline silicon if quartz crucibles with a height of morethan approximately 250 mm, in particular more than approximately 300 mmand quite particularly preferably more than 350 mm are used. Thesingle-zone jacket heater with a defined adjustable temperature profileover the jacket height is of particular advantage during the productionof fluoride monocrystals and germanium monocrystals if crystals with aheight of more than approximately 200 mm are produced.

According to the invention, the heat output of the jacket heater can besuitably adjusted by simple measures, such as, for example, thevariation of the geometric cross section of the jacket heater. Inparticular, in this way, the jacket heater can be simply matched to thegeometry-induced thermal properties of the crucible.

Preferably, the crucible has a polygonal cross section, quiteparticularly preferably, a rectangular or square-shaped cross section,so that polygonal, in particular rectangular or square-shaped elements,preferably, silicon elements, can be cut out of the silicon ingot withan advantageously low amount of wastage. The device according to theinvention is therefore based on a departure from the conventionalconcept of using a rotationally symmetrical melting crucible for theproduction of multicrystalline silicon. Unlike the prior art, the heaterdisposed around the crucible has the same contour as the crucible. Forexample, a square-shaped crucible is therefore surrounded by asquare-shaped heater. The conventional thermal insulation layer betweenheater and crucible is omitted.

According to a further embodiment, the heat output of the single-zonejacket heater decreases in the longitudinal direction of the cruciblegoing from the top to the bottom in correspondence with the temperaturegradient in the centre of the crucible. In particular, the heat outputof the jacket heater decreases per length unit in exactly the same ratioat which the temperature gradient in the centre of the crucibledecreases. According to the invention, this exact, in particularproportional, reproduction of the temperature gradient in the centre ofthe crucible over the entire circumference of the latter is a simple wayof ensuring planar phase interfaces between already crystallised siliconand still molten silicon over the entire cross section of the crucible,in particular also in corner regions of the crucible.

According to a further embodiment, the jacket heater sets or maintains aplurality of planar isotherms vertically to the longitudinal directionof the crucible. The resulting planar phase interface over the entirecross section of the crucible results in an advantageous reduction ofcrystal imperfections and hence to an advantageously low density of etchpits of silicon wafers produced according to the invention.

According to a further embodiment, the distance between the cruciblewall and a plane spanned by the jacket heater over the entirecircumference of the crucible is constant. This measure enables theavoidance of local overheating of regions of the crucible wall thatwould otherwise result in the distortion of the phase interface. Inparticular, in this way, the jacket heater can be disposed uniformlyover the entire circumference of the crucible. According to thisembodiment, the jacket heater also comprises a polygonal cross section,in particular according to a preferred embodiment a rectangular orsquare-shaped cross section, which deviates significantly from theconventional rotationally symmetrical configuration according to theprior art.

In particular in the case of crucibles with rectangular or square-shapedcross sections, increased heat losses have been observed due to a largerradiating surface per unit volume. Increased thermal radiation losses ofthis kind also occur in a milder form with polygonal crucibles withcross sections that are not rectangular or square-shaped. To compensatefor unwanted increased losses of this kind, the heat output of thejacket heater is higher in corner areas of the crucible or alternativelya distance between the crucible wall and the jacket heater in the cornerareas of the crucible may be selected smaller. In this regard, the heatoutput of the jacket heater can be increased continuously or in one ormore discrete steps in the corner areas. Alternatively, the distancebetween the crucible wall and the jacket heater can be reducedcontinuously or in one or a plurality of discrete steps. In particular,the jacket heater can be embodied as continuously curved in the cornerareas, with a minimum distance on an imaginary extension of a lineextending from the centre of the crucible to the respective corner ofthe crucible, wherein this minimum distance is less than it is in theregions of the crucible wall outside the respective corner region.

According to a further embodiment, in particular in the case ofcrucibles with rectangular or square-shaped cross sections, the jacketheater comprises heating elements disposed around the side faces of thecrucible which have a meandering course in the longitudinal direction ofthe crucible or perpendicular thereto. In this way, a comparativelyuniform heat impingement on the crucible wall is achieved and theelectronic layout of the jacket heater can nevertheless be varied in asimple way in correspondence with the temperature gradient in themelting crucible. In this regard, a gap width between the webs of themeandering course of the jacket heater is expediently selected so thatthe efficiently heat-conducting graphite crucible wall itself leads tosufficient smoothing of the temperature profile. The gap width betweenwebs of the jacket heater therefore also depends in particular on thethermal conductivity of the material or materials of the inner crucible,for example the quartz crucible, and on the outer support crucible, forexample the graphite crucible. Expediently, the gap width is in thisregard selected in such a way that any inhomogeneity of the temperatureprofile on the wall of the crucible induced thereby is less than apredetermined temperature deviation, which is preferably less than forexample 5 K, more preferably less than for example 2K and still morepreferably less than for example 1 K.

According to a first embodiment, the heating elements are configured asrectangular webs extending perpendicular to the longitudinal direction,which in the longitudinal direction of the crucible have a meanderingcourse and whose conductor cross sections decrease from the top end tothe bottom end of the crucible in several discrete steps. A jacketheater disposed in this way can be formed by the simple connection ofpreshaped individual parts made of graphite or by casting a suitableheat conducting material in a suitable geometric configuration.

Expediently in this regard, the webs of the jacket heater extend with ameandering course equidistantly and parallel to each other. The websextending horizontal or vertically to the longitudinal direction therebydefine isotherms which extend over the entire circumference of thecrucible at the same height level and hence lead automatically to theformation of planar, horizontal phase interfaces in the crucible. Thedirection of the course of the webs is in this regard inverted ininversion zones lying opposite to the corner areas of the crucible. Thegeometry of the inversion zones, in particular their conductor crosssections, therefore provides a simple parameter for selectivelyspecifying the thermal conditions in the corner areas of the crucible.

In particular, in the case of crucibles with rectangular orsquare-shaped cross sections, the jacket heater comprises heatingelements disposed around the side faces of the crucible which have ameandering course in the longitudinal direction of the crucible orvertically thereto. In this way, a comparatively uniform heatimpingement on the crucible wall is achieved and the electronic layoutof the jacket heater can nevertheless be varied in a simple way incorrespondence with the temperature gradient in the melting crucible. Inthis regard, a gap width between the webs of the meandering course ofthe jacket heater is expediently selected so that the efficientlyheat-conducting graphite crucible wall itself leads to sufficientsmoothing of the temperature profile. The gap width between webs of thejacket heater is therefore also determined in particular by the thermalconductivity of the material or materials of the inner crucible, (e.g.quartz crucible), and the outer support crucible (e.g. graphitecrucible). Expediently, the gap width is hereby selected in such a waythat any inhomogeneity of the temperature profile on the wall of thecrucible induced thereby is less than a predetermined temperaturedeviation, which is preferably less than for example 5 K, morepreferably less than for example 2K and still more preferably less thanfor example 1 K.

In particular, in the case of crucibles with rectangular orsquare-shaped cross sections, special measures may be provided in theregion of the corners in order to guarantee the desired horizontalisotherms there as well. In the region of the diagonals in the inversionzones, without further measures to reduce the conductor cross section,simple inversions in the form of vertical heating webs in the case ofotherwise horizontally meandering heating webs can result in a locallyincreased conductor cross section and hence in reduced heat output withthe consequence of a lower surface temperature on the heater. This meansthat an isothermal behaviour along the longitudinal coordinate of thecrucible could no longer be guaranteed. There would then be an unwanteddecrease in temperature at the corners with negative impacts (stressesin the corners, high defect density and microcracks induced therebyresulting in yield losses). According to the invention, there is avariety of possible measures to compensate such deviations from thedesired conductive equilibrium (isothermal behaviour) along thelongitudinal coordinate. The distance between the crucible wall and thejacket heater in the corner areas of the crucible can be reducedcontinuously or in several steps since the requirement for conductiveequilibrium in principle only exists in the crystallisation phase. Inparticular, the jacket heater can be configured as continuously curvedin the corner areas, with a minimum distance on an imaginary extensionof a line from the centre of the crucible to the respective corner ofthe crucible, wherein this minimum distance is less than it is in theregions of the crucible wall outside the relevant corner region

According to a preferred further embodiment, a conductor cross sectionof webs at the inversion zones of the meandering course is constrictedin the diagonal direction in such a way that it is equal to theconductor cross section of the web before or after the respectiveinversion zone. This results in the maintenance of the electricalresistance and hence in the same heat output or surface temperature inthe inversion zone of the webs as in the region of the horizontallyextending webs.

According to a further embodiment, the constrictions of the conductorcross section at the inversion zones is achieved in a controlled mannerby a plurality of perforations or recesses in or out of the webmaterial, which are disposed in a transverse distribution relative tothe conductor cross section. In this way, the geometry and thedimensions of the perforations or recesses enable the conductor crosssection or the electrical resistance in the inversion zones to matchthat of the webs. The direction of the course of the perforations orrecesses in this regard represent variants which can all lead to thehomogenisation of the horizontal temperature distribution over thecircumference of the crucible in each height coordinate. With an overallrectangular course of the webs, the perforations or recesses can extendin particular along one of the diagonals connecting the corner regionsof the webs. Overall, it is expedient for the plurality of perforationsor recesses to extend mirror-symmetrically or almostmirror-symmetrically about an imaginary mirror axis in the centre of thegap between two adjacent webs.

According to a second embodiment of the present invention, the heatingelements are disposed as rectangular webs extending in the longitudinaldirection whose conductor cross section increases continuously or in aplurality of discrete steps going from the top end to the bottom end ofthe crucible. In this regard, all the webs extending in the longitudinaldirection or perpendicular thereto are embodied the same so that, viewedin the longitudinal direction of the crucible, a plurality of planar,horizontal isotherms or isothermal lines are defined in a quasicontinuous or discrete way by the jacket heater. In this regard, asdescribed above, the gap width between the webs is selected so that theefficiently heat-conducting material of the crucible ensures sufficienthomogenisation of the temperature profile between the webs of the jacketheater. In each case, the regions between the webs do not result indeviations from the monotonous and almost linear temperature rise in theincreasing longitudinal coordinate of the crucible, wherein herelocations where the material to be crystallised comes into contact withthe inner crucible wall are respectively considered.

According to a further embodiment, the jacket heater is made fromindividual segments, which can be optionally, for example in the case oflocal damage or if the jacket heater is to be disposed differently,removed and replaced by another segment. A modular design of this kindhas in particular proved its value for jacket heaters comprising aplurality of heating webs with a meandering course. In this regard, thesegments must be connected in such a way that an unimpeded current flowis ensured at the connecting points or junctures, which necessitatescertain compromises with the choice of the type of connection and thematerials. In particular, the segments may be connected detachably toeach other by means of connecting elements, such as for example wedgesor stoppers or pins with an identical or slightly higher coefficient ofthermal expansion or by means of other positive-locking,friction-locking or non-positive-locking elements, in particular screwsor rivets. According to another embodiment, the segments can also beconnected firmly bonded to each other, for example by soldering orwelding.

According to a further preferred embodiment, no further thermalinsulation is provided between the crucible wall and the jacket heater.It is advantageous with the same cross section of the jacket heater forit to be possible to pull the crucible wall closer to the jacket heaterthus making it possible to produce silicon ingots with a larger crosssection with the same crystallisation system base.

As explained above, a further aspect of the present invention relates toa method for the production of multicrystalline silicon using thevertical-gradient-freeze method (VGF method) using a single-zone jacketheater by means of which the temperature distribution in the crucible isset so that the isotherms of the melting temperature of the silicon tobe grown intersect the melting crucible horizontally. Hereby, slowcooling of the entire furnace in conjunction with a displacement of thevertical temperature profile upward achieves oriented solidification ofthe silicon with the direction of the course of the crystalline regionsin the vertical direction.

To minimise crystal imperfections, care must be taken in this regard toensure that ideally no radial heat flow occurs. While this isconventionally achieved by providing the most ideal possible thermalinsulation surrounding the jacket surface of the crucible, in order toprevent radial heat flow, according to the invention the temperaturegradient in the crucible is simulated by the jacket heater disposedaround the circumference of the crucible. According to the invention,therefore, only reduced efforts are required for the thermal insulationof the system components comprised of the crucible and heater againstthe outer wall of the crystallisation system and hence against theenvironment which results in rapid heat transfer due to the narrowcoupling. The possibilities for controlling the process are, therefore,greatly improved due to the lower time delays.

A further aspect of the present invention relates to the use of acrystallisation system as described above or a correspondingcrystallisation method for the production of multicrystalline silicon bymeans of a vertical-gradient-freeze crystal growth method (VGF), inparticular as a starting material for multicrystalline Si wafers for usein photovoltaics.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe attached drawings from which further features, advantages andobjects to be achieved will be seen. These show:

FIG. 1 in a schematic cross-sectional view, a device for the productionof multicrystalline silicon according to the present invention;

FIG. 2 in a schematic overview, a jacket heater with a meandering courseof the heating webs;

FIG. 3 a in a schematic representation, measures for constriction of theconductor cross section according to a first embodiment of the presentinvention;

FIG. 3 b measures for constriction of the conductor cross sectionaccording to a second embodiment of the present invention;

FIG. 3 c measures for constriction of the conductor cross sectionaccording to a third embodiment of the present invention;

FIG. 4 a-4 c in a schematic overview, different types of connectingmeans for connecting webs of the jacket heater according to FIG. 2;

FIG. 4 d in a perspective view, a further type of connection; and

FIG. 5 a measured temperature profile over a juncture with the type ofconnection according to FIG. 4 d.

In the drawings, the same reference numbers indicate elements or groupsof elements which are identical or which perform a substantiallyequivalent technical function.

DETAILED DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENTS

FIG. 1 shows an example of a vertical-gradient-freeze crystallisationsystem comprising a crucible with a quadrangular cross section.According to FIG. 1, the crucible is formed from a quartz crucible 2,which, for support, is accommodated as a tight fit in a correspondinglyshaped graphite container 4. Therefore, the silicon 3 accommodated inthe crucible 2 does not come into contact with the graphite container 4.The crucible is disposed standing upright so that the crucible wallsextend along the direction of gravity. Above and below the crucible,there is an upper heater 6 or a bottom heater 5, respectively, whereindisposed between the crucible and the bottom heater 5 there is acrucible mounting plate 40, made for example from graphite, which in thedrawing is only schematically indicated. In this regard, the actualholder for the aforementioned crucible is disposed so that a narrow gapis formed between the bottom heater 5 and the crucible mounting plate 40supporting the crucible. The core zone of the crucible is surrounded bya jacket heater 7, i.e. a planar, flat heating device, which will bedescribed in more detail below. With the VGF crystallisation method, allthe heaters 5-7 are temperature-controlled. For this, the surfacetemperatures of the heaters are determined by pyrometers 9 a-9 c at asuitable point, as shown by way of example in FIG. 1, and entered into acontrol unit, which controls or regulates the constant current flowingthrough the heaters 5-7.

To crystallise out the silicon melt, the bottom heater 5 and the upperheater 6 are regulated in such a way that the upper heater 6 is kept ata temperature above the melting temperature of the silicon to beprocessed and the bottom heater 5 is first brought to a temperature justbelow the melting temperature of the silicon to be processed. This firstresults in crystallisation on the bottom of the crucible. Since thebottom heater 5 extends over the entire area of the bottom of thecrucible, the silicon crystallises in not only the centre but on theentire bottom of the crucible in the form of a plurality ofcrystallites. Then, the temperature of each of the three heaters shownis reduced in parallel with that of the other heaters so that the meltin the crucible can solidify continuously upward, wherein the phaseinterface between the already crystallised and the still molten materialextends horizontally, i.e. perpendicular to the direction of gravity.

According to FIG. 1, no further thermal insulation is provided betweenthe crucible wall 2, 4 and the jacket heater 7. Instead, as will bedescribed in more detail below, according to the invention, a suitablegeometric configuration of the jacket heater 7 ensures that thetemperature gradient established by the upper heater 6 and the bottomheater 5 in the crucible is supported or maintained by the heat outputfrom the jacket heater. To this end, the heat output from jacket heateris not locally constant but decreases in the longitudinal direction ofthe crucible going from the top end to the bottom end and namely incorrespondence with the temperature gradient in the centre of thecrucible during the gradual solidification of the silicon melt.

FIG. 2 shows a jacket heater segment according to a first embodiment ofthe present invention comprising a plurality of heating webs with arectangular profile, which form a meandering course in the longitudinaldirection of the crucible. To be more precise, each jacket heatersegment according to FIG. 2 is disposed at a constant distance to acrucible wall in such a way that the webs 10-13 extend exactlyhorizontally, perpendicular to the longitudinal direction of thecrucible. The course direction of the webs 10-13 is inverted at theinversion zones 15-17. According to FIG. 2, the cross section of webs10-13 increases going from the top end to the bottom end of the cruciblein discrete steps. The heat output of the uppermost web 10 is hence thegreatest and decreases in discrete steps, as determined by the conductorcross sections of the webs 11, 12, to the lowest heat output determinedby the cross section of the bottommost web 13.

In an alternative embodiment (not shown), the widths of the webs 10-13are constant, but their thickness increases, viewed perpendicular to theplane of projection in FIG. 2, in discrete steps going from the top endto the bottom end of the crucible.

A constant current flows through a jacket heater comprising a pluralityof jacket heater segments. In this regard, the horizontally extendingwebs 10, 11, 12 and 13 define isotherms (isothermal planes), whichextend over the entire width of the crucible. A plurality of such jacketheaters according to FIG. 2 are disposed around the circumference of thecrucible with the same spacing in each case so that the isotherms set bythe webs 10-13 extend over the entire cross section of the crucible inorder in this way to establish planar, horizontal isothermal surfaces.

Even though in FIG. 2, the jacket heater 7 has four transverse websoverall, according to the invention any other numbers of heating webscan be used. The optimum number of heating webs is determined by thedesired homogenisation of the temperature profile in the crucible and onthe crucible wall. The embodiment of the jacket heater is in this regarddetermined in particular by the width of the gap 14 a-14 c between thewebs 10-13, the selected distance between the jacket heater 7 and thecrucible wall and the thermal properties of the crucible wall. Theefficiently heat-conducting graphite crucible 4 (see FIG. 1) with anadequate thickness and the quartz crucible located therein lead in thisregard to a certain smoothing of the vertical temperature profile. Theabove parameters are selected so that the position of one web of thejacket heater on the temperature profile at the interface between thesilicon and lateral internal wall of the quartz crucible cansubstantially no longer be determined.

Generally, with the jacket heater according to FIG. 2 with a length ofwebs 1, a width of the webs b_(i) (wherein i designates the runningindex for the web) and a thickness d (vertical to the plane ofprojection in FIG. 2), the electrical resistance of a heating web withthe index i is described by:

Ri˜1/Ai, whereinAi=bi×d.

Then, the following applies to the cross-sectional area:A1<A2<A3<A4.

From this, the following applies to the resistances of the individualmeanders:R1<R2<R3<R4.

Consequently:T1>T2>T3>T4.

Therefore, in the vertical direction, a temperature profile is obtainedwith a temperature increasing in discrete steps upward. When a constantcurrent intensity flows through the heating meander, a lower temperatureis generated in the webs with a large cross section (corresponding to alow electrical resistance) than in the webs with a small cross section(corresponding to a high electrical resistance).

As is easily evident to a person skilled in the art, the variation ofthe conductor cross section through which current flows from web to webcan also be achieved by varying the web thickness d instead of the webwidth b, as described above.

In an exemplary embodiment, according to FIG. 2 the following arearatios are established. A1/A1 1 A2/A1 1.055 A3/A1 1.11 A4/A1 1.165

These area ratios produce the following resistance ratios: R1/R1 1 R2/R10.948 R3/R1 0.901 R4/R1 0.858

As is evident from FIG. 2, the width of the heating conductor alsovaries in the inversion zones 15 to 17 in a corresponding way. The widthof the inversion zone 15 is hence less than the width of the inversionzone 16, which is in turn less than the width of the inversion zone 17.The variation of the widths of the inversion zones follows thetemperature profile to be formed.

If one considers the inversion zones 15-17 of the jacket heater 7according to FIG. 2, local cross section enlargements occur in thematerial through which current flows. Without countermeasures, thesewould result in a low temperature at the corner areas of the crucible.According to the invention, this is counteracted by the selectiveconstriction of the conductor cross section in the inversion zones. Inparticular, such a constriction of the conductor cross section can alsocompensate increased heat losses in the corner areas of the crucible,for example due to higher thermal radiation losses caused by the largerradiating area per unit of volume.

According to FIG. 3 a, a plurality of perforations or recesses 18 isdisposed along the diagonals of the respective inversion zone and to beprecise aligned on the diagonals. Overall, the perforations or recesses18 are disposed mirror-symmetrically to the centre line of the gap 14 a.Obviously, it is also possible to provide a plurality of such rows ofperforations or recesses. The disposal and choice of the number ofperforations or recesses can be used to establish the suitableresistance ratio between the web 10, 11 extending in a horizontaldirection and the associated inversion zone.

With the embodiment according to FIG. 3 b, rectangular recesses aredisposed along the diagonals. The choice of the ratio s/b can be used toestablish an optimum resistance ratio.

According to FIG. 3 c, constriction recesses are disposed along thediagonals wherein, disposed between the recesses 20, there is a concaveinwardly curved course of the edge. The above recesses 11, 20 can inparticular be formed by milling from the material of the heatingconductor.

Preferably, the webs of the jacket heater are made of graphite. Sinceaccording to the invention, crucibles with a base of 680×680 mm or evenlarger crucibles are used and correspondingly large graphite blocks forthe production of webs of the jacket heater are either not available atall or are only available at a comparatively high price, according to afurther embodiment, the webs of the jacket heater segments are formed,as described below with reference to FIG. 4 a to 4 d, once again from aplurality of smaller segments. In this regard, care must be taken toensure that the current flow through the junctures between the jacketheater segments and between the smaller segments is as unimpeded aspossible. For this, positive-locking engaging junctures with rectangulargeometry are used.

According to FIG. 4 a, the ends of the heating segments 100, 101 aresubstantially L-shaped so that a graduated interface 102 is formedbetween the two segments 100, 101. According to FIG. 4 b, a centralU-shaped recess is disposed at the end of the segment 100 and disposedat the opposite end of the segment 101 is an inversely U-shapedprojection 103, which fits tightly into the recess of the segment 100.As a result, an interface 102 with a central projection forms betweenthe segments 100, 101. According to FIG. 4 c, disposed at the ends ofsegments 100, 101, there is a rectangular recess to accommodate aconnecting element 104.

FIG. 4 d shows the connection according to FIG. 4 a in a perspectiveoverview, wherein the segments 100, 101 are penetrated by cylindricalconnecting elements 104. The connecting elements 104 can be made of thematerial used for segments 100, 101. The engagement of the connectingelements 104 in the segments 100, 101 can be positive-locking,friction-locking or non-positive locking. The connecting elements 104can alternatively also be made of another material with an identical orslightly higher coefficient of thermal expansion than the material usedfor segments 100, 101.

Exemplary Embodiment 1

Two rectangular heater segments made of graphite are connected togetherin the manner according to FIG. 4 d and a temperature profile wasmeasures along the dotted line according to FIG. 4 d with localresolution. For reasons of corrosion, the measurements were taken in anormal air atmosphere and at a lower temperature than the subsequentoperating temperature under current throughput. The measured homogeneityof the temperature profile at this low temperature level is, howevercompletely transferable to the subsequent higher operating temperaturelevel.

As may be seen from FIG. 5, the temperature fluctuations in theconnecting region or juncture are of an order of magnitude of less thanapproximately ±5° Celsius.

Exemplary Embodiment 2

To produce a multicrystalline silicon ingot, the interior of a meltingcrucible is filled with a lumpy or granular silicon feedstock. To removeunwanted atmospheric oxygen, the device according to FIG. 1 is rinsedwith inert gas, for example argon. Following this, the melting of thesilicon can commence under vacuum or even at normal pressure bypositioning and powering up the upper heater, bottom heater and thejacket heater. After several hours a temperature above the meltingtemperature and less than 1550° C. is achieved and the melting iscompleted. The bottom heater is now reduced to a defined temperature ofat least 10° C. below melting temperature. The initiation of the crystalgrowth now takes place on the bottom of the melting crucible. After ashort time, an equilibrium temperature profile is established and theinitiated crystal growth comes to a stop. In this condition, the upperheater and bottom heater have the desired temperature difference, whichis equal to the temperature difference between the top and bottom end ofthe jacket heater. Now, one of the heaters is powered down and to beprecise in parallel to the others. There is a columnar growth of aplurality of crystals. Corresponding to the horizontal phase interface,the growth takes place vertically from the bottom upward. Themulticrystalline Si ingot obtained in this way is then cooled to roomtemperature and removed. In this way, a square-shaped Si ingot of680×680 mm is obtained. The multicrystalline silicon ingot has a lowcrystal defect density over the entire crystal volume.

As is automatically evident to a person skilled in the art, thesegmented meandering heater design can also be used for the heatersabove and below the crucible. However, there is expediently no variationof the current-carrying sections since the top side and bottom side ofthe silicon ingot should be heated as homogeneously as possible. Theoptional heater provided under the base of the crucible assists themelting of lumpy silicon with the object of the shortest possibleprocess time. However, in principle, the heater on the base of thecrucible is not required during crystallisation.

A heater above the crucible also assists in reducing the process timefor melting lumpy silicon when this takes place in the crucible. Duringcrystallisation, the function of the heater above the crucible is, inconjunction with the jacket heater, to reduce the temperature level inthe entire crucible so that the crystallisation always takes place on aplanar phase interface and, to be precise, independently of the heightin the crucible at which it takes place. The temperature reduction ofthe heaters is in this regard electronically controlled and occurswithout any lowering of the crucible.

The heater design in conjunction with the electronically controlledtemperature reduction achieves in particular the following advantages:

-   -   the planar phase interface in all crystallisation phases enables        a columnar, vertical growth of the Si grains with a homogeneous        structure;    -   low number of linear defects in the ingot, identifiable on the        Si wafer from a lower density of etch pits;    -   minimisation of the convection flows in the still molten Si        above the phase interface and thereby minimisation of the        transport of Si₃N₄ particles from the internally coated quartz        crucible wall into the interior of the melt or minimisation of        the transport of SiC particles from the surface of the molten Si        into the interior of the melt resulting in fewer occlusions in        the ingot; the yield and the efficiency are improved by the        aforementioned minimisation;    -   prevention of stresses in the corner area of the ingot and        thereby avoidance of increased defect concentrations in the        corners, avoidance of stress-induced micro-cracks which would        otherwise result in yield losses in later processing steps.

LIST OF REFERENCE NUMBERS

-   1 Crystallisation system-   2 Crucible-   3 Melt-   4 Graphite container-   40 Crucible mounting plate-   5 Bottom heater-   6 Upper heater-   7 Jacket heater-   8 Thermal insulation-   9 Temperature sensor-   10 Horizontal web-   100 First segment-   101 Second segment-   102 Interface-   103 Central projection-   104 Connecting element-   11 Horizontal web-   12 Horizontal web-   13 Horizontal web-   14 a-c Gap-   15 Vertical connecting web-   16 Vertical connecting web-   17 Vertical connecting web-   18 Hole/recess-   19 Recess-   20 Recess-   21 Edge

1. A device for the production of monocrystalline or multicrystallinematerials using the vertical-gradient-freeze-method (VGF method),comprising a fixed crucible having a bottom end and a top end, if viewedin the longitudinal direction, said fixed crucible having a polygonalcross section, and a heating device for melting the silicon, wherein thedevice is configured to form a temperature gradient in the longitudinaldirection, in the crucible, and the heating device comprises a flatheating element to suppress a heat flow perpendicular to thelongitudinal direction, said flat heating element being disposed aroundthe crucible and comprising a plurality of heating elements disposed onside faces of the crucible with a meandering course in the longitudinaldirection or vertically thereto, in which device, in inversion zones ofthe meandering course in corner areas of the crucible, a heat output ofthe flat heating element is higher or the distance between the cruciblewall and the flat heating element is lower.
 2. The device according toclaim 1, wherein the heat output of the flat heating element in thecorner areas is constant or increased in at least one discrete step orwherein the distance between the crucible wall and the flat heatingelement is reduced in at least one discrete step.
 3. The device toaccording to claim 1, wherein the heating elements are embodied asrectangular webs, wherein a conductor cross section of the webs at theinversion zones is constricted in a diagonal direction in such a waythat it is equal to the conductor cross section of an associated webbefore or after the respective inversion zone.
 4. The device accordingto claim 3, wherein constrictions of the conductor cross section at theinversion zones are formed by a plurality of perforations or recesses inor out of the web material, which are disposed in a transversedistribution relative to the conductor cross section.
 5. The deviceaccording to claim 4, wherein the perforations or recesses are alignedin the diagonal direction.
 6. The device according to claim 4, whereinthe perforations or recesses are disposed mirror-symmetrically to acentre line of a gap formed between two adjacent heating elements. 7.The device according to claim 3, wherein the webs extend perpendicularto the longitudinal direction, and their conductor cross sectionsincrease in discrete steps going from the top end to the bottom end andwherein the webs extend equidistantly and parallel to each other.
 8. Thedevice according to claim 3, wherein the webs extend in the longitudinaldirection and their conductor cross section increases continuously or ina plurality of discrete steps going from the top end to the bottom endand wherein the webs extend equidistantly and parallel to each other. 9.The device according to claim 1, wherein the external contour of theflat heating element is shaped in correspondence with the externalcontour of the crucible so that a distance between the flat heatingelement and the crucible is constant.
 10. The device according to claim1, wherein the heat output of the flat heating element decreases in thelongitudinal direction in correspondence with the temperature gradientin the centre of the crucible.
 11. The device according to claim 1,wherein the flat heating element is configured to set or maintain aplurality of planar isotherms perpendicular to the longitudinaldirection.
 12. The device according to claim 1, wherein the flat heatingelement forms a heating zone which is configured so that the heat outputdecreases going from the top end to the bottom end in order to at leastcontribute to the temperature gradient formed in the crucible.
 13. Thedevice according to claim 3, wherein the webs comprise a plurality ofsegments, which are connected detachably to each other by means ofconnecting elements or firmly bonded to each other.
 14. The deviceaccording to claim 1, wherein no thermal insulation is provided betweenthe crucible wall and the flat heating element.
 15. The device accordingto claim 1, wherein no thermal insulation is provided between thecrucible wall and the flat heating element.
 16. The device according toclaims 1, wherein said multicrystalline material is multicrystallinesilicon, said fixed crucible having a rectangular or square-shaped crosssection.
 17. A method for the production of monocrystalline ormulticrystalline materials using the vertical-gradient-freeze-method(VGF method) in a fixed crucible having a bottom end and a top end, ifviewed in the longitudinal direction, said fixed crucible having apolygonal cross section, wherein a heating device forms a temperaturegradient going from the top end to the bottom end in the crucible andwherein a flat heating element disposed around the crucible suppresses aheat flow perpendicular to the longitudinal direction, said flat heatingelement comprising a plurality of heating elements, which are disposedon side faces of the crucible and have a meandering course in thelongitudinal direction or perpendicular thereto, in which method heatlosses in corner areas of the crucible are compensated by increasing aheat output of the flat heating element or by decreasing the distancebetween the crucible wall and the flat heating element at inversionzones of the meandering course in corner areas of the crucible.
 18. Themethod according to claim 17, wherein the heat output of the flatheating element in the corner areas is increased continuously or in atleast one discrete step or the distance between the crucible wall andthe flat heating element is reduced in at least one discrete step. 19.The method according to claim 17, wherein the heating elements areprovided as rectangular webs having a conductor cross section which isconstricted at the inversion zones in the diagonal direction in such away that it is equal to the conductor cross section of an associated webbefore or after the inversion zone.
 20. The method according to claim19, wherein the constrictions of the conductor cross section at theinversion zones are formed by a plurality of perforations or recesses inor out of the web material, said perforations or recesses being formedin a transverse distribution relative to the conductor cross section.21. The method according to claim 20, wherein the perforations orrecesses are aligned in the diagonal direction.
 22. The method accordingto claim 20, wherein the perforations or recesses are disposedmirror-symmetrically to a centre line of a gap formed between twoadjacent heating elements.
 23. The method according to claim 19, whereinthe webs are provided as webs extending perpendicular to thelongitudinal direction, whose conductor cross sections increase indiscrete steps going from the top end to the bottom end, and wherein thewebs are provided extending equidistantly and parallel to each other.24. The method according to claim 19, wherein the webs are provided aswebs extending in the longitudinal direction, whose conductor crosssections increase continuously or in a plurality of discrete steps goingfrom the top end to the bottom end, and wherein the webs are providedextending equidistantly and parallel to each other.
 25. The methodaccording to claim 17, wherein the crucible and the flat heating elementare provided such that a distance between the crucible wall and a planespanned by the flat heating element is constant over the entirecircumference of the crucible.
 26. The method according to claim 17,wherein the heat output of the flat heating element is reduced in thelongitudinal direction in correspondence with the temperature gradientin the centre of the crucible.
 27. The method according to claim 17,wherein the flat heating element sets or maintains a plurality of planarisotherms perpendicular to the longitudinal direction.
 28. The methodaccording to claim 17, wherein the heat output of the flat heatingelement forming a heating zone is reduced going from the top end to thebottom end in order to at least contribute to the temperature gradientformed in the crucible.
 29. The method according to claim 18, whereinthe webs are provided with a plurality of segments, wherein the segmentsare connected detachably to each other by means of connecting elementsor the segments are provided firmly bonded to each other.
 30. The methodaccording to claim 17, wherein no thermal insulation is provided betweenthe crucible wall and the flat heating element.
 31. The method accordingto claim 17, wherein said multicrystalline material is multicrystallinesilicon grown in a said fixed crucible having a rectangular orsquare-shaped cross section.